Sf{11 {0 system with constant density and constant pressure differential maintaining means for a high-voltage switchgear

ABSTRACT

Metal clad switch gear for high voltages comprising switches and accessories, low-overpressure and high-overpressure compartments filled with SF6-gas and a gas-controlsystem controlling the flow of gas to and from said compartments and keeping both the pressure difference between said compartments and the density of the SF6-gas in the low-overpressure compartment(s) substantially constant.

United States Patent Inventor Rintje Boersma Harmelen, Netherlands Appl. No. 815,674 Filed Apr. 14, 1969 Patented Oct. 19, 1971 Assignee N.V. COQ

Utrecht, Netherlands Priority Apr. 22, 1968 Netherlands 68.05638 SF,- SYSTEM WITH CONSTANT DENSITY AND CONSTANT PRESSURE DIFFERENTIAL MAINTAINING MEANS FOR A HIGH-VOLTAGE SWITCHGEAR 7 Claims, 5 Drawing Figs.

US. Cl 200/148 E, 200/ 148 B Int. Cl H0lh 33/56 Field of Search 200/ 148 R,

[56] References Cited UNITED STATES PATENTS 3,129,309 4/1964 McKeough et al 200/148 B 3,423,553 1/1969 Pratsch 200/148 B 3,280,288 10/1966 Frowein 200/148 (.2) 3,303,310 2/1967 McKeough et a1... 200/148 (.2) 3,359,390 12/1967 Frowein 200/148 (.5) 3,390,241 6/1968 Frowein 200/148 (.5) 3,405,249 10/1968 Wonkim 200/84 X FOREIGN PATENTS 938,165 10/1963 Great Britain 200/148 (.5)

Primary Examiner-Robert K. Schaefer Assistant Examiner-Robert A. Vanderhye Altorneylmirie, Smiley, Snyder & Butrum ABSTRACT: Metal clad switch gear for high voltages comprising switches and accessories, low-overpressure and highoverpressure compartments filled with SF gas and a gas-controlsystem controlling the flow of gas to and from said compartments and keeping both the pressure difference between said compartments and the density of the SF gas in the lowoverpressure compartment(s) substantially constant.

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R INTJ'E BOERSMA ATTORNEYS PATENTEDUDT 19 new SHEET 3 OF 3 ATTORNEYS SF SYSTEM WITH CONSTANT DENSITY AND CONSTANT PRESSURE DIFFERENTIAL MAINTAINING MEANS FOR A HIGH-VOLTAGE SWITCI'IGEAR The invention relates to metal clad switch gear for high voltages comprising a metal envelope adapted to be connected to earth and containing at least a gas blast circuit breaker, a lowpressure compartment filled with SF,,gas of lower overpressure for the insulation of the live parts of the switch gear from earth and a high-pressure compartment filled with SF -gas of higher overpressure for the extinction of the switching arc and a composite gas-control system to control the flow of gas to and from said compartments, said gas-control system being arranged to keep the pressure difference between both compartments substantially constant by controlling the density of the sF -gas in each one of the two compartments.

' ln switch gear filled with SF -gas changes of the state of the gas economy occur repeatedly as a result of the properties of said gas and the operation of the switch gear. In switch gear local temperature variations can occur which are produced by the current flowing through the conductors and the continuously varying temperature of the outer air. These temperature variations can have, during operation, a range of 25 C. to +85 C. For the extinction of the switching arc and, if necessary, for driving the switches gas from the high-pressure compartment is used and either directly or by a roundabout way returned to the low-pressure compartment, Moreover, the change of pressure of a SF -gas mass of a given volume due to a change of temperature is greater as the density of said mass is higher, which means that the isochores in the pressure-versus-temperature diagram are steeper at higher density than at lower density. Within the range of operation of the switch gear the pressure and the temperature of the SF,-gas can have values, at which said gas condenses. The gas-control system must reckon with these properties and also ensure that the density of the SF -gas in no one of the compartments containing live parts falls down below a given value, since otherwise the insulation from earth is endangered, and that the pressure difference between the high-pressure compartment and the low-pressure compartment remains above the minimum value required for a sufiicient extinguishing action and, if necessary, for a satisfactory driving of the switches.

Switch gear of the described kind, in which the various above mentioned facts are reckoned with, has been disclosed in US. Pat. 3,390,241. In said switch gear the gas-control system tries to keep both the total quantity of SF -gas contained in the switch gear and the pressure difference between the high-pressure gas mass and the low-pressure gas mass constant as much as possible. This means, that not only during the switching process, but also at a rise in temperature in the stationary state of the switch gear SF -gas must be transported from the high-pressure compartment towards the low-pressure compartment, so that the pressure in the low-pressure compartment then will increase more than would have been the case, when in the low-pressure compartment only a rise in temperature occurs. This additional increase of pressure in the low-pressure compartment has the disadvantage that the envelope of said compartment, which is considerably larger than that of the high-pressure compartment, must be made stronger and that also the sealing and the packing between the separable parts of the envelope of the low-pressure compartment must be made proof against higher pressures. This sealing may give difficulties in view of the large dimensions of the joints to be sealed between said parts of the envelope. The stronger envelope and the heavier sealing make the switch gear more expensive. The attention is drawn to the fact that an increase of the density of the sF -gas in the low-pressure compartment has no real advantage regarding to the safe state of insulation in the switch gear, since the minimum density of the gas in the low-pressure compartment, said density obtaining in the known switchgear at the lowest temperature to be expected, say a temperature of 25 C., guarantees already a sufficiently safe state of insulation.

The invention has for its object to provide switchgear of the kind referred to which is simpler, has a cheaper construction, can be easily made gastight and for which a relatively simple gas-control system is required. It consists in that the gas-control system is so arranged and so operates as to keep also, under all circumstances, the density of the SF -gas in the lowpressure compartment substantially constant. This measure has the advantage that at a rise in temperature the pressures in the low-pressure compartment and in the high-pressure compartment will increase relatively slightly, so that lighter structures can be used and a satisfactory seal can easily be realized. Moreover, a relatively simple gas-control system can be used, since the control has only to do with two quantities, viz the density in the low-pressure compartment and the said pressure difference.

It is observed that switchgear filled andoperating with SP gas and comprising highand low-pressure compartments, in which the density of the SF -gas in the low-pressure compartment is kept constant, has been disclosed by the U.S. Pat. specification 3,129,309. Also in this switchgear the density of the SF -gas in the high-pressure compartment is kept constant. This results in that the pressure difference between both compartments and in consequence thereof the switching process strongly depend on the temperature and in that at great rises in temperature high pressures will obtain in the high-pressure system.

' Advantageously, the switchgear can be so constructed that the low-pressure compartment is connected to a discharge conduit and a supply conduit, said conduits cooperating with means acting on the density of the gas in said compartment, said means ensuring that gas flows from and to the low-pressure compartment, when the density of the gas in said compartment becomes higher and lower than the required density,

respectively, whereas a conduit provided with a valve and a conduit with a compressor are connected between the lowpressure compartment and the high-pressure compartment, both compartments being connected to a differential-pressure relay which opens said valve or starts said compressor when the pressure difference between both compartments becomes higher or lower than the required pressure difl'erence, respectively. In this switchgear only a relay acting on the density of the SF -gas contained in the low-pressure compartment and a differential-pressure relay are necessary. The high-pressure compartment can only receive gas from or supply gas to the low-pressure compartment, so that the gas supply of the switchgear takes place only through the low pressure compartment.

As SF -gas is too expensive to be discarded, switchgear is recommended in which gas discharged from the switchgear is collected in a storage tank. The switchgear and its gas-control system may then be constructed in such a manner that the discharge conduit is connected through a check valve closing towards the low-pressure compartment with a low-pressure storage tank and the supply conduit is connected through a valve to a high-pressure storage tank, whereas the two storage tanks are interconnected by a conduit provided with a second compressor and a relay acting on the density of the gas in the low-pressure compartment is used said relay opening and closing said valve, when the density of the gas in the low-pressure compartment becomes a given value lower than or becomes equal to the required density, respectively. In addition, means are provided which start the second compressor, when the low-pressure storage tank contains too much gas for temperature in said tank. Owing to the use of storage tanks in the gascontrol system no SF,-gas will be lost except by leakage. By using a high-pressure storage tank in the gas circuit a relatively large stock 7 of SF -gas can be stored in a relatively small volume and it becomes possible to supply the insulating compartments of the switchgear very quickly with gas, when therein the density of the gas, for whatever reason, falls down below a value which is dangerous to the state of insulation.

If each switching unit of the switchgear has a gas-control system provided with its own gas storage tanks, this system may advantageously be made in such a manner, that the relay acting on the density of the gas in the low-pressure compartrneritstafts'and stops'thesecond compressor, when the den si ty fact that the temperature of the gas in the storage tank, said gas having the temperature of the outer air, can differ considerably from that of the gas in the low-pressure compartment, through which the main current heating the gas passes. At a change of this temperature difference, said change resulting in a change of the pressure difierence between the lowpressure compartment and the low-pressure storage tank, the gas from the low-pressure compartment could circulate continuously through the two storage tanks. The differential-pressure relay prevents the unnecessary circulation of the gas in the low-pressure system.

The switchgear can be made considerably simpler, when the gas control systems of a plurality of switching units cooperate with a common low-pressure storage tank, a common highpressure storage tank and a conduit connected between these storage tanks which is provided with a second compressor. In that case the differential-pressure relay can be omitted. However, also a relay acting on the density of the gas in the lowpressure storage tank is then provided, said latter relay starting and stopping the second compressor at density values which are so much higher than the density values, at which the relay acting on the density of the gas in the low-pressure compartment closes and opens the valve, as to guarantee that at the maximum temperature difference to be expected between the gas in the low-pressure compartment and the gas in the low-pressure storage tank the second compressor is started at a higher gas pressure than that, at which the valve is closed, and is stopped at a higher gas pressure than that, at which the valve is opened. If the density relays in the low-pressure compartments of the switching units are adjusted in this manner in respect of the density relay provided in the common low-pressure storage tank, the gas systems of said switching units cannot influence one another.

If the gas blast circuit breaker is provided with an expansion tank to collect the used extinguishing gas, it is advantageous to connect the expansion tank through a conduit having a check valve closing towards said tank and a filter to the low-pressure storage tank and through a conduit having a check valve closing towards the low-pressure compartment to said compartment. During the switching process the extinguishing gas, which is deionized in the expansion tank and cleansed by the filter, will then reach the low-pressure compartment not directly but by a roundabout way, so that in the low-pressure compartment a sudden rise of the density will not be produced, since said rise of density is neutralized relatively slowly by the control system. Moreover, the used highly heated extinguishing gas will get sufficient time to cool down, before it is returned into the switchgear. Preferably, a buffer tank is inserted in the conduit extending between the expansion tank and the low-pressure storage tank, said bufi'er tank being provided between the check valve and the filter. This buffer tank damps the velocity of the extinguishing gas which escapes from the expansion tank during the switching-off process, so that it flows slowly through the filter and can be well cleansed. The conduit having the check valve and extending between the low-pressure compartment and the expansion tank ensures that in the stationary state of the switchgear the pressure in the expansion tank is about equal to that in the low-pressure compartment.

If at a predetennined temperature and pressure of the SF,- gas condensation should occur in the high-pressure compartment, the pressure in said compartment would remain constant, notwithstandingthe supply of gas from or the dischzgge thereof to the low-pressure compartment, as long as condensate is to be found in the high-pressure compartment. in that case it is not possible to keep the pressure difference between the high-pressure compartment and the low-pressure compartment constant, when the circumstances change. In order to avoid this difficulty the high-pressure compartment can be connected by a condensate conduit and a vapor conduit to an evaporator adapted to be heated by a heating element and provided with a relay acting on the liquid level in the evaporator, said relay putting the heating element into operation, when said level exceeds a predetermined value. By this measure the SF -gas contained in the high-pressure compartment is heated above its dew point.

The invention will be further elucidated with the aid of the accompanying drawing. Therein is:

FIG. 1 a temperature-versus-pressure diagram of SF -gas at constant volume. for the range which is interesting to switch gear constructed according to the invention, said F K]. 1 showing lines of constant density (isochores),

F l6. 2 a schematic view of the part of the switchgear which is necessary to understand the invention, said switchgear being provided with a gas-control system carried out in accordance with the invention,

FIG. 3 a similar schematic view of an other embodiment of the gas-control system of said switchgear,

FIG. 4 a schematic view of a variant of the gas-control system shown in FIG. 3 and FIG. 5 a pressure-versus-temperature diagram to determine the adjustment of density relays used in the gas-control system shown in FIG. 4.

From the diagram shown in FIG. I, in which the pressure of the sF -gas has been plotted vertically and the temperature thereof horizontally, it appears that at constant volume the pressure of the gas rises less at lower density than at higher density. The lines 1 of constant density (isochores) thus show a greater steepness as the density of the SF -gas is higher.

The metal clad switchgear illustrated in FIGS. 2 and 3 comprise a gas blast circuit breaker 3 accommodated in a highpressure compartment 2 filled with SF -gas, said circuit breaker having four places of interruption to interrupt the main current circuit, an isolator switch 4 connected in series with said circuit breaker and adapted to keep the main current circuit interrupted and to close said circuit, two bus bar isolators 5, 6 connected in series with the gas blast circuit breaker 3, two rails 7, 8 forming part of different bus bar systems, a cabel terminal box 9, an earthing switch 10, an overvoltage protective device 11 and current transformers 12, 13 for measuring and protection.

The container enclosing the high-pressure compartment 2 and accommodating the gas blast circuit breaker 3 is positioned in a low-pressure compartment 14 also filled with SF gas and surrounded by a metal envelope adapted to be connected to' earth. Mounted in said compartment 14 are also the isolator switch 4, the movable contacts of the bus bar isolator switches 5,6, the current transformers l2, l3 and an expansion tank 15, in which the extinguishing gas from the highpressure compartment 2 and used to extinguish the switching arcs during the switching-off process is collected and deionized. The fixed contacts of the bus bar isolators 5, 6 the rails 7, 8, the cabel terminal box 9, the earthing switch 10, the overvoltage protective device 11 and the connecting conductors extending between the isolator switch 4 and the cabel terminal box 9 and between said box and the overvoltage protective device 11 are accommodated in individual low-pressure compartments l6, 17, 18, 19, 20, 21, 22, 23, 24 filled with SF -gas, said latter compartments communicating in a not shown manner with the low-pressure compartment 14, so that therein the same pressure obtains and all these compartments may be considered as one single low-pressure compartment.

The SF -gas contained in the low-pressure compartment serves mainly to insulate the live parts from earth. The SF -gas contained in the high-pressure compartment 2 is in the first place used to extinguish the switching arcs but it may also be used for the pneumatical driving of the gas blast circuit breaker and the isolator switches 4, 5, 6.

According to the invention the density of the SF -gas in the low-pressure compartment 14 and, consequently, also in the other low pressure compartments is kept constant as much as possible, e.g. at a value of 40 gram per liter. This value has been indicated in the diagram shown in FIG. 1 by the thick isochore 25. In addition, the density in the high-pressure compartment 2 is so controlled, as to keep also the pressure dif ference between the high-pressure compartment 2 and the low-pressure compartment 14 substantially constant, e.g. at a value of kg./cm. In order to maintain this pressure difference under allcircumstances the density of the gas contained in the high-pressure compartment must be changed in accordance with the thick line 26 shown in the diagram given in FIG. 1 at a change of temperature.

For the desired gas economy in the switchgear a gas-control system has been provided which in the embodiment shown in FIG. 2 is carried out and operates as follows:

The low-pressure compartment 14 is connected bothto a supply conduit 29 provided with an electromagnetically controlled valve 28 and connected with a gas bottle 30 and to a discharge conduit 32 provided with an electromagnetically controlled valve 31. Connected between the low-pressure compartment 14 and the high-pressure compartment 2 are a conduit 33 provided with an electromagnetically controlled valve 34 and a conduit 35 provided with a compressor 36. The expansion tank is connected to the low-pressure compartment 14 both by means of a conduit 37 provided with two filters 38, which can be cleaned alternately, and by means of a conduit 39 provided with a check valve 40 closing towards the low-pressure compartment 14. The high-pressure compartment 2 is connected by means of a condensate conduit 41 and a vapor conduit 42 with an evaporator 44 adapted to be heated by an electric heating element 43.

For controlling the gas-control system a relay 45 acting on the density of the SF -gas, contained in the low-pressure cornpartment l4, and having contacts 46, 47 for the electric control of the valves 28, 31 is provided in the low-pressure compartment 14 and a differential-pressure relay 48 having contacts 49, 50 for the electric control of the valve 34 and the compressor 36 is connected to the low-pressure compartment 14 and the high-pressure compartment 2. Moreover, mounted in the evaporator 44 is a relay 51 acting on the level of the condensate, collected therein, and having a contact 52 for the control of the heating element. The control is effected .as follows:

In the stationary state of the switchgear the gas in the lowpressure compartment 14 has the required density and the gas in the high-pressure compartment 2 has such a density as to guarantee that the pressure difference between the compartments 2 and 14 has the required value. The pressure in the expansion tank 15 is equal to that in the low-pressure compartment 14.

During operation of the switchgear different changes of the conditions may occur. If the temperaturs in the switchgear rises, the pressure in the high-pressure compartment 2 will increase more than that in the low-pressure compartment 14 (see the diagram shown in FIG. 1). In that case the pressure difference between both compartments will become greater than desired. This has the effect that the differential-pressure relay 48 comes into action and closes the contact 49, so that the valve 34 is opened. Gas from the high-pressure compartment 2 then flows through the conduit 33 to the low-pressure compartment 14 till the required pressure difference has been restored. The gas fed to the low-pressure compartment 14 increases the density of the gas in said compartment, which has the result that the density relay 45 comes into action and closes the contact 47. Due thereto the valve 31 is opened, so

that the excess gas can flow away from the low-pressure compartment 14 through the conduits 27 and 32. Should the temperature in the switchgear fall down, the pressure in the highpressure compartment 2 will fall further down than that in the low-pressure compartment 14. Also in that case the relay 48 comes into action, so that the contact 50 is'closed, the compressor 36 is put into operation and gas is pumped from the low-pressure compartment 14 to the high-pressure compartment 2. Owing thereto the density in the low-pressure compartment 14 decreases, the relay 45 comes into action, the contact 46 is closed and the valve 28 is opened, so that gas from the gas bottle 30 is supplied to the low-pressure compartment 14 to supply the gas deficiency in said compartment.

Not only a change of temperature but also the switching-off process upsets the stationary state. During the switching-off process gas flows from the high-pressure compartment 2 through the switching contacts of the gas blast circuit breaker 3 to the expansion tank 15 and thereafter from this tank through the conduit 37 and a filter 38 to the low-pressure compartment 14. This results in that the pressure difference between the compartments 2 and 14 decreases and the density in the low-pressure compartment 14 increases. The differential-pressure relay 48 then closes the contact 50, whereby the compressor 36 is started. Also the density relay 45 comes into action and closes the contact 47. However, the closing of this contact 47 does not result in opening the valve 31 of the discharge conduit 32, since this contact is connected in series with an auxiliary contact 53 of the differential-pressure relay 48, said auxiliary contact 53 being closed in the state of equilibrium of the relay 48 and in the position thereof, in which the contact 49 is closed, but being opened in the position of the relay 48, in which the contact 50 is closed and the compressor 36 operates. In that case no gas is lost through the discharge conduit 32 notwithstanding the density in the compartment 14 is too high.

At a predetennined pressure and a predetermined temperature within the pressure-temperature range, in which the switchgear operates, the SF -gaS can condense. This danger only occurs in the high-pressure compartment 2. As the condensation of the gas and the evaporation of the condensate have a disturbing effect on maintaining the pressure difference between the highand low-pressure compartment the heating element 43 is set into operation as soon as condensate flows into the evaporator 44. Due thereto the SF -gas in the highpressure compartment 2 is heated to get a temperature above the dew point.

As SF -gas is too expensive to be discharged to the open air the gas-control system shown in FIG. 3 is provided with a lowpressure storage tank 54, the discharge conduit 32 being connected to said storage tank 54 through a check valve 55 closing towards the low-pressure compartments 14 of the switchgear. The expansion tank 15 is connected to the lowpressure storage tank 54 through the conduit 37, a check valve 56 closing towards the low-pressure compartment 14, a buffer tank 57 and the filters 38. The low-pressure storage tank 54 is connected to a high-pressure storage tank 60 by a conduit 58 providedwith a second compressor 59, said highpressure storage tank 60 being connected to the supply conduit 29 provided with the electromagnetically operating valve 28. The gas bottle 30 is connected with the high-pressure storage tank 60 by a conduit 61 provided with a valve 62 and it has the task to cover losses of SF -gas by leakage. In this case the differential-pressure relay 48 is connected to the low-pressure compartment 14 through the expansion tank 15 and the conduit 39 provided with the check valve 40. This has the ad vantage that the differential-pressure relay 48 becomes earlier active and also starts the compressor 36, during the switchingoff process, earlier than it does in the system shown in FIG. 2, since, during a switching-off operation, the pressure in the ex pansion tank 15 rises much quicker than the pressure in the low-pressure compartment 14.

The relay 45 acting on the density of the gas in the lowpressure compartment 14 controls by its contact 46 the valve 28 provided in the supply conduit 29 and by its contact 47 the second compressor 59. Finally a second differential-pressure relay 63 is provided between the low-pressure compartment 14 and the low-pressure storage tank 54, the contact 64 of said relay controlling the compressor 59. This gas-control system operates as follows:

lf, due to a rise in temperature in the switchgear, the pressure in the highpressure compartment 2 becomes too high in respect of that in the low-pressure compartment 14 the valve 34 is opened by the relay 48. This has the effect that gas flows from the compartment 2 to the compartment 14 and from the latter through the conduit 32 and the check valve 55 to the low-pressure storage tank 54. In that case the density in the low-pressure compartment 14 increases, so that the relay 45 closes its contact 47 and starts the compressor 59. This compressor sucks the gas excess fromthe low-pressure compartment l4 and the low-pressure storage tank 54 and pumps it into the high-pressure storage tank 60.

If, due to a drop of temperature in the switchgear, the pressure in the high-pressure compartment 2 becomes too low'in respect of that in the low-pressure compartment 14 the relay 48 starts the compressor 36. Owing thereto the compartment 2 receives gas from the compartment 14 and the density of the gas in the low-pressure compartment 14 decreases and becomes too low, so that the relay 45 closes its contact 46 resulting in that the valve 28 is opened and gas flows from the high-pressure storage tank 60 to the low-pressure compartment 14 to supply the gas deficiency. Since no gas can flow from the low-pressure storage tank 54 to the low-pressure compartment 14 the pressure in the tank 54 will temporarily become higher than that in the compartment 14, so that, depending' on the sensibility of the relay 63, the contact 64 is closed and thereby the compressor 59 is started. The latter continues to operate till the moment, at which the pressures in the compartment 14 and the tank 54 will be equal again. From that moment on the low-pressure compartment 14 and, through this compartment, the conduit 27, 32, the check valve 55 and the low-pressure storage tank 54 are fed with gas from the high-pressure storage tank 60 till the gas in the low-pressure compartment 14 has reached the right density.

in the stationary state of the switchgear the same pressure obtains in the expansion tank 15 as in the low-pressure compartment l4 owing to the connection through the conduit 39 provided with the check valve 40. During the switching-off process the pressure in the expansion tank rises considerably and the pressure in the high-pressure compartment 2 falls down. The result thereof is that the relay 48 comes immediately into action and it starts the compressor 36 over the contact 50. The density of the gas in the low-pressure compartment 14 then decreases, so that also the relay 45 is put in action and it opens the valve 28 by means of its contact 46. Due thereto gas flows from the high-pressure storage tank 60 to the low-pressure compartment 14. Owing to the fact that at first-the density and the pressure in the low-pressure compartment 14 decrease, as well as due to the fact that the extinguishing gas flows from the expansion tank 15 through the conduit 37, the check valve 56, the buffer tank 57 and a filter 38 to the lowpressure storage tank 54, so that the pressure in the tank 54 rises, the differential-pressure relay 63 comes into action and the contact 64 is closed, whereby the compressor 59 is started and the gas excess is pumped away from the low-pressure storage tank 54 to the high-pressure storage tank. As soon as the right pressure difference between the high-pressure compartment 2 and the low-pressure compartment 14 has been restored and the density in this compartment 14 has again reached the right value, the relays 45, 48 and 63 return to their rest positions, the compressors 36 and 59 are stopped and the valve 28 is closed.

The storage tanks 54 and 60 are situated outside the switchgear and no electric current flows through them, so that great temperature differences can be set up between the gas in these tanks and the gas in the switchgear. Said temperature differences could disturb the stationary gas economy in the system. If the switchgear has been dead for a long time the temperature in the switchgear and that in the lowpressure storage tank 54 will be equal, so that the gas densities in the compartment 14 and the tank 54 will also be equal, since the pressures are equal too. However, if current flows through the switchgear the temperature in the low-pressure compartment 14 will become higher than that in the low-pressure storage tank 54. This results in that, if the pressuredoes not change, the density in the storage tank 54 will become higher and the density in the low-pressure compartment 14 will become lower. Consequently, there will be no gas transport from the compartment 14 to the storage tank 54. If the density in the compartment 14 becomes too low the relay 45 will come into action, the contact 46 will be closed and the valve 28 will be opened, so that the gas deficiency in the low-pressure compartment 14 will be supplied from the high-pressure storage tank 60. It is also possible that in the considered case, due to the decrease of the density in the compartment 14, the pressure difference between the high-pressure compartment 2 and the low-pressure compartment 14 makes the relay 48 operative, so that the valve 34 is opened, which will last till the moment, at which the required pressure difference has been restored. From that moment on the pressure in the low-pressure compartment 14 continues to rise due to the supply of gas from the storage tank 60, so that the relay 48 comes into action and starts the compressor 36 again till, at the right density in the low-pressure compartment 14, the right pressure difference between the compartments 2 and 14 has again been reached. If after the termination of this control action the temperature difference between the gas in the switchgear and the gas in the storage tank 54 decreases again, either owing to an unintended heating of the tank 54 or, which is more probable, owing to the fact that the switchgear becomes currentles's, the density in the low-pressure compartment 14 will not change but there will be set up a pressure difference between the storage tank 54 and said compartment 14, said pressure difference putting the relay 63 into operation so that the compressor 59 is started and the pressure in the tank 54 is made equal again to that in the low-pressure compartment 14. The same control occurs when the temperature in the storage tank 54 would, for whatever reason, become higher than that in the low-pressure compartment 14.

By the right adjustment of the sensibility of the relays 45, 48 and 63 it is avoided that the control becomes instable. The sensibility of these relays defines, within which range the den sity in the low-pressure compartment 14 and the pressure difference between the compartments 2 and 14 can be kept constant. It will be obvious, that it is not possible to keep these quantities exactly constant.

The buffer tank 57 has the object to level the sudden pressure rise in the expansion tank 15 due to the switching-off of the gas blast circuit breaker 3, so that the extinguishing gas which is contaminated by the switching process flows with low speed through the filter 38 to the low-pressure storage tank 54. The effect of the filter is thereby considerably increased.

In the switchgearshown in FIG. 3 each switching unit is provided with its own set of storage tanks 54, 60 and accessories. In many cases it is more advantageous to use one single lowpressure storage tank and one single high-pressure storage tank for a plurality of switching units. The gas-control system then must be so arranged, that change of the conditions in a given switching unit does not affect the conditions in other switching units of the switchgear. An example of switchgear consisting of a plurality of switching units which are connected in parallel to a set of storage tanks 54, 60 is illustrated in FIG. 4.

The gas-control system in H6. 4 differs from that shown in FIG. 3 in that the difi'erential-pressure relay 63 has been' at the higher temperature T, this closing takes place at the higher pressure indicated by point 68. The density relay 45 closes the contact 46, when the pressure in the low-pressure compartment 14 becomes a value equal to one-half kg./cm.

lower than that defined by the isochore 25. Consequently, at the temperature T the valve 28 is opened, when the gas pressure in the compartment 14 has reached the value indicated by point 69 but at the temperature T the valve 28 will open at the pressure indicated by the point 70. The density relay 65 in the low-pressure storage tank 54 has been adjusted at higher density values. At the temperature T the contact 66 of the relay 65 is closed and the second compressor 59 is started, when the density in said storage tank has reached a value, at which the pressure in said tank has the value indicated by the point 71. The contact 66 is opened and the compressor 59 is stopped again, when the pressure in the storage tank 54 is returned to the pressure indicated by the point 72, said pressure being, in the present case, one-half kg./cm. lower than the pressure at the point 71 which starts the compressor. Now, the adjustment of the density relay 65 is so chosen as to render the density values indicated by the parallel lines 73 and 74, at which the second compressor 59 is started and stopped, respectively, so much higher than the density values given by the isochore 2S and the line 75 parallel thereto, at which the valve 28 is closed and opened, respectively, that even at the maximum difference to be expected of say 15 C. between the lower temperature of the gas in the low-pressure storage tank 54 and the higher temperature of the gas in the low-pressure compartment 14 the relay 65 puts the compressor 59 in operation at a pressure which is higher than the pressure, at which the relay 45 closes the valve 28, and stops the compressor 59 at a pressure, which is higher than the pressure at which the relay 45 opens the valve 28. This means, that in the given example, in which the temperatures T and T, differ the expected maximum value of 15 C. under normal operational circumstances, the point 71 comes to lie higher than the point 68 and the point 72 comes to lie higher than the point 70. Only, when the relays 45 and 65 are thus adjusted, the control of the gas-control system of each switching unit is stable, which will appear from the following:

If one starts from the state, in which a switching unit is dead, the gas in the low-pressure compartment 14 of said unit has the right density defined by the isochore 25 (FIG. and the temperature of the gas in the low-pressure compartment 14 is equal to the temperature of the gas in the low-pressure storage tank 54, that means is equal to T the gas densities and the gas pressures in said compartment 14 and said storage tank 54 will be equal too. The gas pressure then is given by the point 67. If thereafter the switching unit is switched in, so that current flows through it, the temperature in the low-pressure compartment 14 could, in the most unfavorable situation, rise l5 C. with respect to the value T so that the pressure in said compartment 14 will rise to the point 68 and gas will flow from the compartment 14 through the conduit 32 and the check valve 55 to the storage tank 54 till the pressure in the tank 54 has become equal to that in the compartment 14 again. The pressure in the compartment 14 then is somewhat lower than that indicated by the point 68 and the pressure in the tank 54 then is somewhat higher than that indicated by the point 67. Should, for whatever reason, a further loss of gas occur from the compartment 14, so that the pressure indicated by the point 70 is reached, the valve 28 will be opened by the relay 45 till the pressures in the compartment 14 and the storage tank 54 have again reached the value indicated by the point 68. Since the point 71, at which the compressor 59 is put in action, lies higher than the point 68 said compressor will remain inoperative.

If, owing to the switching-ofi of the gas blast circuit breaker 3, gas flows from the high-pressure compartment 2 to the lowpressure storage tank 54 the pressure therein can exceed the value indicated by the point 71, so that then the compressor 59 is started and gas is pumped from the low-pressure storage tank 54 to the high-pressure storage tank 60 till the pressure in the low-pressure storage tank 54 (that means also the pressure in the low-pressure compartment 14) has reached the value indicated by the point 72. As this value lies above the value indicated by the point 70, at which the valve 28 is opened by the relay 45, said valve remains closed. It follows therefrom, that the compressor 59 will not be put into operation when the valve 28 is in its open position and also that the valve 28 will not be opened, when the compressor 59 is in operation, so that the instable situation, in which the valve 28 is open and the compressor 59 is in operation, whereby gas would be continuously pumped from the low-pressure storage tank 54 to the high-pressure storage tank 60 and hence to the low-pressure compartment 14 and through the conduit 32 back to the lowpressure storage tank 54, can not occur.

If one has to reckon with a higher maximum temperature difference than say l5 C. the adjustment of the density relay 65 and that of the density relay 45 must differ more, that means the lines 73 and 74 will have to lie higher in respect of the lines 25 and 75 than has been illustrated in F IG. 5.

What we claim is: 1. A switchgear system for high voltage electrical circuits, comprising in combination;

an electrically conductive envelope housing at least a gas blast circuit breaker and related portions of a high voltage electrical circuit, said envelope being adapted for connection to earth and defining low-pressure and high-pressure compartments of fixed volumes, said low pressure compartment beingfilled with an amount of gaseous sulfur hexafluoride sufficient to establish a relatively low pressure in such compartment which will insulate live parts of the switchgear from earth, and said high-pressure compartment being filled with an amount of gaseous sulfur hexafluoride sufiicient to establish a relatively high pressure in such compartment which is capable of extinguishing a switching are caused by operation of said circuit breaker; means for maintaining a substantially constant density of gas in said low-pressure compartment whereby the lowpressure compartment is subjected to limited gas pressure variation in response to temperature variations; and

means for varying the density of the gas in said high-pressure compartment to maintain a substantially constant gas pressure differential between said high-pressure and low-pressure compartments.

2. Metal clad switchgear as claimed in claim 1 wherein said means for maintaining a substantially constant density of gas in said low-pressure compartment comprises a discharge conduit (32) and a supply conduit (29), said conduits (32, 29) being connected to the low-pressure compartment (14) and cooperating with flow regulating means (45, 31, 28) acting on the density of the gas in said compartment (14), said flow regulating means ensuring that gas flows from and to the lowpressure compartment 14) when the density of the gas in said compartment (14) becomes higher and lower than the required density, respectively, said means for varying the density of the gas in said high-pressure compartment comprising a conduit (33) provided with a valve (34) and a conduit (35) provided with a compressor (36), said conduits being connected between the low-pressure compartment (14) and the high-pressure compartment (2), and a differentialpressure relay (48, 49, 50) connected to both compartments (2, 14) and either opening said valve (34) or starting said compressor (36), when the pressure difference between both compartments (2, 14) becomes higher or lower than the required pressure difi'erence, respectively (FIGS. 2, 3, 4).

3. Metal clad switchgear as claimed in claim 2, comprising a check valve (55) closing towards the low-pressure compartment (14) and being provided in the discharge conduit (32) 1 l which is connected to a low-pressure storage tank (54), the supply conduit (29) being connected through a valve (28) to a high-pressure storage tank (60), a conduit (58) provided with a second compressor (59) being connected between both storage tanks (54, 60), a relay (45) acting on the density of the gas in the low-pressure compartment (14) and adapted to open and to close said valve (28) when the density of the gas in the low-pressure compartment 14 becomes a given value lower than or becomes equal to the required density, respectively, and means (63, 65) for starting the second compressor (59), when the low-pressure storage tank (54) contains too much gas at the temperature obtaining in said tank (FIG. 3,

4. Metal clad switchgear as claimed in claim 3, in which the relay (45) acting on the density of the gas in the low-pressure compartment (14) starts and stops the second compressor (59) when the density of the gas in said compartment (14) becomes higher or a given value lower than the required density, respectively, a differential-pressure relay (63) being provided which is connected both to the low-pressure compartment (l4) and the low-pressure storage tank (54) and is adapted to start the second compressor (59), when the pressure in the low-pressure compartment (14) (FIG. 3).

5. Metal clad switchgear as claimed in claim 3, comprising also a relay (65) acting on the density of the gas in the lowpressure storage tank (54), said relay starting and stopping the second compressor (59) at density values which are so much higher than the density values, at which the relay (45) acting on the density of the gas in the lower-pressure compartment (14) closes and opens the valve (28), that at the maximum temperature difference to be expected between the gas in the lower-pressure compartment (14) and the gas in the lowerpressure storage tank (54) the second compressor (59) is started at a higher gas pressure than that, at which the valve (28) is closed, and is stopped at a higher gas pressure than that, at which the valve (28) is opened (FIG. 4).

6. Metal clad switchgear as claimed in claim 3, comprising a gas blast circuit breaker provided with an expansion tank (15) to collect the used extinguishing gas, said expansion tank (15) communicating with the low-pressure storage tank (54) through a conduit (37) provided with a check valve (56) closing towards said tank and a filter (38), said expansion tank (15) also communicating with the low-pressure compartment (14) through a conduit (39) provided with a check valve (40) closing towards said compartment l4).

7. Metal clad switchgear as claimed in claim 6, comprising a buffer tank (57) provided between the check valve (56) and the filter (38) in the conduit (37) extending between the expansion-tank (l5) and the low-pressure storage tank (54). 

1. A switchgear system for high voltage electrical circuits, comprising in combination; an electrically conductive envelope housing at least a gas blast circuit breaker and related portions of a high voltage electrical circuit, said envelope being adapted for connection to earth and defining low-pressure and high-pressure compartments of fixed volumes, said low pressure compartment being filled with an amount of gaseous sulfur hexafluoride sufficient to establish a relatively low pressure in such compartment which will insulate live parts of the switchgear from earth, and said high-pressure compartment being filled with an amount of gaseous sulfur hexafluoride sufficient to establish a relatively high pressure in such compartment which is capable of extinguishing a switching arc caused by operation of said circuit breaker; means for maintaining a substantially constant density of gas in said low-pressure compartment whereby the low-pressure compartment is subjected to limited gas pressure variation in response to temperature variations; and means for varying the density of the gas in said high-pressure compartment to maintain a substantially constant gas pressure differential between said high-pressure and low-pressure compartments.
 2. Metal clad switchgear as claimed in claim 1 wherein said means for maintaining a substantially constant density of gas in said low-pressure compartment comprises a discharge conduit (32) and a supply conduit (29), said conduits (32, 29) being connected to the low-pressure compartment (14) and cooperating with flow regulating means (45, 31, 28) acting on the density of the gas in said compartment (14), said flow regulating means ensuring that gas flows from and to the low-pressure compartment (14) when the density of the gas in said compartment (14) becomes higher and lower than the required density, respectively, said means for varying the density of the gas in said high-pressure compartment comprising a conduit (33) provided with a valve (34) and a conduit (35) provided with a compressor (36), said conduits being connected between the low-pressure compartment (14) and the high-pressure comparTment (2), and a differential-pressure relay (48, 49, 50) connected to both compartments (2, 14) and either opening said valve (34) or starting said compressor (36), when the pressure difference between both compartments (2, 14) becomes higher or lower than the required pressure difference, respectively (FIGS. 2, 3, 4).
 3. Metal clad switchgear as claimed in claim 2, comprising a check valve (55) closing towards the low-pressure compartment (14) and being provided in the discharge conduit (32) which is connected to a low-pressure storage tank (54), the supply conduit (29) being connected through a valve (28) to a high-pressure storage tank (60), a conduit (58) provided with a second compressor (59) being connected between both storage tanks (54, 60), a relay (45) acting on the density of the gas in the low-pressure compartment (14) and adapted to open and to close said valve (28) when the density of the gas in the low-pressure compartment 14 becomes a given value lower than or becomes equal to the required density, respectively, and means (63, 65) for starting the second compressor (59), when the low-pressure storage tank (54) contains too much gas at the temperature obtaining in said tank (FIG. 3, 4).
 4. Metal clad switchgear as claimed in claim 3, in which the relay (45) acting on the density of the gas in the low-pressure compartment (14) starts and stops the second compressor (59) when the density of the gas in said compartment (14) becomes higher or a given value lower than the required density, respectively, a differential-pressure relay (63) being provided which is connected both to the low-pressure compartment (14) and the low-pressure storage tank (54) and is adapted to start the second compressor (59), when the pressure in the low-pressure compartment (14) (FIG. 3).
 5. Metal clad switchgear as claimed in claim 3, comprising also a relay (65) acting on the density of the gas in the low-pressure storage tank (54), said relay starting and stopping the second compressor (59) at density values which are so much higher than the density values, at which the relay (45) acting on the density of the gas in the lower-pressure compartment (14) closes and opens the valve (28), that at the maximum temperature difference to be expected between the gas in the lower-pressure compartment (14) and the gas in the lower-pressure storage tank (54) the second compressor (59) is started at a higher gas pressure than that, at which the valve (28) is closed, and is stopped at a higher gas pressure than that, at which the valve (28) is opened (FIG. 4).
 6. Metal clad switchgear as claimed in claim 3, comprising a gas blast circuit breaker provided with an expansion tank (15) to collect the used extinguishing gas, said expansion tank (15) communicating with the low-pressure storage tank (54) through a conduit (37) provided with a check valve (56) closing towards said tank and a filter (38), said expansion tank (15) also communicating with the low-pressure compartment (14) through a conduit (39) provided with a check valve (40) closing towards said compartment (14).
 7. Metal clad switchgear as claimed in claim 6, comprising a buffer tank (57) provided between the check valve (56) and the filter (38) in the conduit (37) extending between the expansion-tank (15) and the low-pressure storage tank (54). 